1. Field of the Invention
The present invention relates to an optical head used for optically recording information on media and/or reproducing and/or erasing the information recorded on the media, which is applicable to optical data recording or storing system using various types of optical disks.
2. Description of the Related Art
In the optical recording field, several types of optical data recording devices or systems using various optical disks, such as magneto-optical (MO) disks and compact disks (CDs), have already been practically used extensively. These optical disks have concentric or spiral recording tracks and pieces of information are optically recorded and reproduced along the tracks.
Moreover, to cope with the recent need to further increase the storage capacity, a new type of optical recording devices using digital video or versatile disks (DVDs) have been developed and put into practice.
On the other hand, there has been the strong need to downsize optical recording apparatuses and to lower their fabrication cost. To meet the need, a technique that simplifies the optical system configuration of optical heads by applying hologram elements to optical heads as one of the base or key components of optical recording apparatuses has been developed and disclosed. Examples of optical heads using this technique were disclosed in the Japanese Non-Examined Patent Publication Nos. 7-9708 published in 1995 and 10-269588 published in 1998.
FIGS. 1 and 2A to 2C show the configuration of the prior-art optical head disclosed in the Japanese Non-Examined Patent Publication No. 7-9708, in which the tracking error detection function is added to the configuration disclosed in this Publication. This head may be termed the “first prior-art optical head” later.
In FIGS. 1 and 2A to 2C, X and Y denote the radial and tangential directions of concentric or spiral tracks of a disk-shaped recording medium, respectively, and 2 denotes the longitudinal axis of the optical system in the prior-art head.
A linearly polarized light beam (i.e., an incident light beam), which is emitted from a laser diode 101 mounted on a heat sink 102, is reflected by a mirror 103 and then, enters a polarizing hologram element 104. The reason why the incident light beam is linearly polarized is to prevent it from diffracting by the element 104. The incident light beam emitted by the laser diode 101 passes through the element 104 without diffraction and then, it is converted to a circularly polarized light beam by a quarter-wave plate 105. The circularly polarized light beam thus generated is then focused by an objective lens 106 to be irradiated on a disk-shaped recording medium 107. The beam thus irradiated forms a specific spot on the medium 107.
The circularly polarized light beam thus irradiated is reflected by the medium 107, at which the beam absorbs a piece of information recorded in the area of the medium 107 corresponding to the spot. The circularly polarized light beam thus reflected passes through the objective lens 106 and then, it is converted to a linearly polarized light beam by the quarter-wave plate 105. The linearly polarized light beam thus generated is diffracted by the hologram element 104, generating not only a reproduction or playback signal beam but also a focusing error signal beam and a tracking error signal beam. The focusing and tracking error signal beams are received by a focusing error signal beam receiver 109 and a tracking error signal beam receiver 110. These two receivers 109 and 110 are formed in an optical detector 108.
The method of detecting the focusing error, tracking error, and reproduction signal beams in the first prior-art head is explained below with reference to FIGS. 2A to 2c. 
The polarizing hologram element 104 is a concentric phase-type diffraction grating made by forming a proton exchange region in a proper crystal such as a lithium niobate (LiNbO3) crystal. As shown in FIG. 2A, the element 104 has arc-shaped gratings whose center is offset with respect to the center of the element 104 itself.
When a reflected light beam 112 by the recording medium 107 passes through the hologram element 104, a zero-order diffracted beam (i.e., a transmitted beam) 113a is not generated while a +1st-order diffracted beam 113b is generated due to a convex lens function and a −1st-order diffracted beam 113c is generated due to a concave lens function.
The focusing error signal beam receiver 109 of the optical detector 108 has three rectangular beam receiving regions 109a, 109b, and 109c. The tracking error signal beam receiver 110 of the detector 108 has eight rectangular beam receiving regions 109a, 110b, 110c, 110d, 110e, 110f, 110g, and 110h. Thus, the detector 108 has eleven beam receiving regions in total.
The total beam receiving surface of the focusing error signal beam receiver 109 is equal in size to that of the tracking error signal beam receiver 110.
The region 109a is equal in size to the region 109c. The region 109b is twice as large in size as the region 109a or 109c. The eight regions 110a to 110h are equal in size to each other, each of which has a half size of the region 109a or 109c. 
The detector 108 is located on the optical axis Z of the first prior-art head in such a way that the zero-order diffracted light beam 113a generated from the reflected light beam 112 correctly focuses on the surface of the detector 108 when the incident light beam correctly focuses on the surface of the medium 107 by the objective lens 106. Thus, when the +1st-order diffracted beam 113b and the −1st-order diffracted beam 113c are received at the middle region between the focusing and tracking error signal beam receivers 109 and 110, these two beams 113b and 113c form equal-sized circular spots on the surface of the middle region.
When the medium 107 approaches the optical head with respect to the focusing point of the objective lens 106 due to surface fluctuation of the medium 107 or the like, the focusing angle of the reflected light beam 112 entering the hologram element 104 decreases, thereby moving the focal point of the diffracted beams 113a, 113b, and 113c to be apart from the lens 106. Therefore, the spot diameter of the beam 113c on the focusing error signal beam receiver 109 increases while the spot diameter of the beam 113b on the tracking error signal beam receiver 110 decreases. Contrarily, when the medium 107 moves to be apart from the optical head with respect to the focusing point of the objective lens 106, the focusing angle of the reflected light beam 112 entering the hologram element 104 increases, thereby moving the focal point of the diffracted beams 113a, 113b, and 113c toward the lens 106. Therefore, the spot diameter of the beam 113c on the focusing error signal beam receiver 109 decreases while the spot diameter of the beam 113b on the tracking error signal beam receiver 110 increases.
Here, as shown in FIG. 2C, the electrical output signals generated by the beam receiving regions 109a, 109b, and 109c of the focusing error signal beam receiver 109 and then, current-to-voltage converted and amplified by corresponding current-to-voltage conversion amplifiers 120 are respectively defined as s109a, s109b, and s109c. Similarly, the electrical output signals generated by the beam receiving regions 110a, 110b, 110c, 110d, 110e, 110f, 110g, and 110h of the tracking error signal beam receiver 110 and then, current-to-voltage converted and amplified by corresponding current-to-voltage conversion amplifiers 120 are respectively defined as s110a, s110b, s110c, s110d, s110e, s110f, s110g, and s110h. 
Then, the focusing error signal FE is produced by using the spot size detection (SSD) method in the following way.
A differential amplifier 121 is electrically connected to the beam receiving regions 109a, 109b, and 109c and the beam receiving regions 110a, 110b, 110c, 110d, 110e, 110f, 110g, and 110h, as shown in FIG. 2C. Therefore, the focusing error signal FE(SSD) is given by the following equation (1).FE(SSD)=(s109a+s109c+s110b+s110c+s110f+s110g)−(s109b+s110a+s110d+s110e+s110h)  (1)
If the recording tracks of the medium 107 are deviated from their desired position due to eccentricity or the like, the radial distribution of the optical strength on the medium 107 varies. Thus, the tracking error signal TE is given by the push-pull (PP) detection method in the following way.
A differential amplifier 122 is electrically connected to the beam receiving regions 110a, 110b, 110c, 110d, 110e, 110f, 110g, and 110h, as shown in FIG. 2C. Therefore, the tracking error signal TE(PP) is given by the following equation (2).TE(PP)=(s110a+s110b+s110c+s110d)−(s110e+s110f+s110g+s110h)  (2)
The tracking error signal TE may be obtained by the differential phase detection (DPD) method, in which a differential amplifier 123 and a differential phase detection circuit 124 are connected, as shown in FIG. 2c. In this case, the tracking error signal TE(DPD) is given by the following equation (3).TE(DPD)=(s110a+s110b+s110g+s110h)−(s110c+s110d+s110e+s110f)  (3)
By using a summing amplifier 125, the information playback or reproduction signal HF is given by the following equation (4).HF−s109a+s109b+s109c+s110a+s110b+s110c+s110d+s110e+s110f+s110g+s110h  (4)
FIGS. 3A to 3C show the configuration of the prior-art optical head disclosed in the Japanese Non-Examined Patent Publication No. 10-269588. The layout of the individual components such as the objective lens 106 is substantially the same as that shown in FIG. 1 and therefore, it is not explained here for the sake of simplification. This head may be termed the “second prior-art optical head” later.
The method of detecting the focusing error, tracking error, and playback signal beams is explained below with reference to FIGS. 3A to 3C.
A polarizing hologram element 130 has four rectangular regions 130a, 130b, 130c, and 130d, which are defined by a division line extending along the x direction (i.e., the radial direction of the recording tracks of the medium 107) and another division line extending along the Y direction (i.e., the tangential direction of the recording tracks of the medium 107). As shown in FIG. 3A, each of the regions 130a, 130b, 130c, and 130d forms a linear phase-type grating whose directions are different from the division lines.
When a reflected light beam 135 by the recording medium 107 passes through the hologram element 130, a zero-order diffracted beam 135a is not generated while four +1st-order diffracted beams 135f, 135g, 135h, and 135i are generated due to a convex lens function and four −1st-order diffracted beams 135b, 135c, 135d, and 135e are generated due to a concave lens function.
A reproducing signal beam receiver 132 of an optical detector 131 has a rectangular beam receiving region. An error signal beam receiver 133 of the optical detector 131 has six rectangular beam receiving regions 133a, 133b, 133c, 133d, 133e, and 133f. Thus, the detector 131 has seven beam receiving regions in total.
The total beam receiving region of the reproducing signal beam receiver 132 is equal in size to that of the error signal beam receiver 133.
The regions 133a, 133b, 133c, and 133d of the error signal beam receiver 133 are equal in size to each other. Each of the region of the reproduction signal beam receiver 132 and the regions 133e and 133f of the error signal beam receiver 133 is sufficiently wide for receiving the corresponding diffracted light beam.
The optical detector 131 is located on the optical axis Z of the second prior-art head in such a way that the zero-order diffracted light beam 135a generated from the reflected light beam 135 correctly focuses on the surface of the detector 131 when the incident light beam correctly focuses on the surface of the medium 107 by the objective lens 106. Each of the diffracted beams 135b, 135c, 135d, 135e, 135f, 135g, 135h, and 135i generated by the hologram element 130 has a sector-like beam shape responsive to the fact that the light 135 is irradiated to the adjoining sector-shaped regions 130a, 130b, 130c, and 130d of the hologram element 130, as shown in FIG. 3A.
The +1st-order diffracted beam 135f generated by the region 130a of the hologram element 130 is irradiated onto the division line of the beam receiving regions 133c and 133d. The +1st-order diffracted beam 135g generated by the region 130b of the element 130 is irradiated onto the division line of the beam receiving regions 133a and 133b. The +1st-order diffracted beam 135h generated by the region 130c of the element 130 is irradiated onto the beam receiving region 133e. The +1st-order diffracted beam 135i generated by the region 130d of the element 130 is irradiated onto the beam receiving region 133f. The −1st-order diffracted beams 135b, 135c, 135d, and 135e generated respectively by the regions 130a, 130b, 130c, and 130d of the element 130 are irradiated onto the beam receiving surface of the reproduction signal receiver 132.
The layout of the beam spots thus formed on the reproduction and error signal beam receivers 132 and 133 is clearly shown in FIG. 3C. Each of these beam spots has a sector-like shape with various orientations.
When the medium 107 approaches the second prior-art optical head with respect to the focusing point of the objective lens 106 due to surface fluctuation of the medium 107 or the like, the focusing angle of the reflected light beam 135 entering the hologram element 130 decreases, thereby moving the focal point of the diffracted beams 135b, 135c, 135d, 135e, 135f, 135g, 135h, and 135i to be apart from the lens 106. Therefore, the spot size of the beams 135f, 135g, 135h, and 135i on the error signal beam receiver 133 increases without changing their sector-like spot shape. Contrarily, when the medium 107 moves to be apart from the optical head with respect to the focusing point of the objective lens 106, the focusing angle of the reflected light beam 135 entering the element 130 increases, thereby moving the focal point of the diffracted beams 135b, 135c, 135d, 135e, 135f, 135g, 135h, and 135i toward the lens 106. Therefore, the spot size of the beams 135f, 135g, 135h, and 135i on the error signal beam receiver 133 decreases without changing their sector-like spot shape.
Here, as shown in FIG. 3C, the electrical output signals generated by the beam receiving regions 133a, 133b, 133c, 133d, 133e, and 133f of the error signal beam receiver 133 and then, current-to-voltage converted and amplified by corresponding current-to-voltage conversion amplifiers 140 are respectively defined as s133a, s133b, s133c, s133d, s133e, and s133f. Similarly, the electrical output signal generated by the reproducing signal beam receiver 132 and then, current-to-voltage converted and amplified by a corresponding current-to-voltage conversion amplifier 120 is defined as s132.
Then, the focusing error signal FE is produced by using the knife edge detection (KED) method in the following way.
Since a differential amplifier 141 is electrically connected to the beam receiving regions 133a, 133b, 133c, and 133d, as shown in FIG. 3C, the focusing error signal FE(KED) is given by the following equation (5).FE(KED)=(s133a+S133d)−(s133b+s133c)  (5)
If the recording tracks of the medium 107 is deviated from their desired position due to eccentricity or the like, the radial distribution of the optical strength on the medium 107 varies. Thus, the tracking error signal TE is given by the push-pull (PP) detection method in the following way.
A differential amplifier 142 is electrically connected to the beam receiving regions 133a, 133b, 133c, 133d, 133e, and 133f, as shown in FIG. 3C. Therefore, the tracking error signal TE(PP) is given by the following equation (6).TE(PP)−(s133a+s133b+s133f)−(s133c+s133d+s133e)  (6)
The tracking error signal TE may be obtained by the differential phase detection (DFD) method, in which a differential amplifier 143 and a differential phase detection circuit 144 are used, as shown in FIG. 3C. In this case, the tracking error signal TE(DFD) is given by the following equation (7).TE(DFD)−(s133a+s133b+s133e)−(s133c+s133d+s133f)  (7)
By using a summing amplifier 145 connected as shown in FIG. 3C, the information reproduction signal HF is given by the following equation (8).HF=s132−s133a−s133b+s133c+s133d+s133e+s133f  (8)
The above-explained first and second prior-art optical heads have three problems described below.
Specifically, the first problem is that the signal processing circuits is complicated and large in scale and at the same time, the electric output signals tend to be degraded in quality due to noises. This is because the count of the beam receiving regions of the optical detector is excessively large, which is explained in detail below.
With the above-explained first and second prior-art optical heads, the focusing error signal is detected through the beam size change of the +1st-order −1st-order diffracted beams generated by the hologram element 104 or 130. Therefore, each of the error signal beam receivers 109, 110, and 133 of the optical detectors 108 and 131 needs to be formed to have three or more beam receiving regions. Also, to detect the tracking error signal simultaneously with the focusing error signal, any one of the error signal beam receivers 109, 110, and 133 needs to be formed to have four beam receiving regions with the division lines extending along the radial and tangential directions (i.e., X and Y) of the disk-shaped recording medium.
Moreover, with the first prior-art optical head shown in FIGS. 1 and 2A to 2C, the optical detector 108 needs to be formed to have eleven beam receiving regions in total. Also, since the output signals of the focusing and tracking error signal receivers 109 and 110 are used for generating both the focusing and tracking error signals, buffer amplifiers (not shown in FIG. 2C) are required for the respective beam receiving regions, thereby increasing the circuit scale. Also, in this case, a lot of necessary amplifiers are configured at several stages and therefore, the quality of the focusing and tracking error signals tends to degrade due to noises occurring in the amplifiers.
The second problem is that the optical detector 131 and the hologram element 130 have to be mounted with high accuracy in the second prior-art optical head. Specifically, the reflected light beam 135 is divided into four parts by the two perpendicular division lines on the hologram element 130, generating the eight diffracted beams 135b to 135f. Thus, if the relative positional relationship between the division lines on the element 130 and the division lines on the optical detector 131 deviates from their desired relationship, the focusing and/or tracking error signal or signals tends to contain some offset and at the same time, the detection sensitivity to the focusing and/or tracking error or errors tends to lower.
The third problem is that complicated positioning operation is necessary to align the optical axes of all the optical components. Specifically, because a light source, optical elements, and optical detectors are integrated on a base, not only the optical components but also the package have to be precisely processed and finished. This raises the fabrication cost of the optical head itself.
Moreover, with the optical heads of this sort, generally, to suppress the bad effect caused by the eccentricity of a recording medium, an objective lens is shifted under control through detection of the tracking error signal. In this case, there arises a problem that some offset occurs in the tracking error signal. Also, to produce the optical heads at sufficiently low cost, there has been the need to form the package of the heads by a proper plastic material. To meet this need, it is essential to efficiently dissipate the heat emitted by a laser diode.